In Okayama Prefecture, when humid air settles over a stream, hotaru—Japan's fireflies—begin signalling. On warm, windless evenings, they often start glowing about two hours after sunset under favourable conditions, inviting silence beside water.

Fireflies aren't flies but beetles, and most species stay near water—ponds, streams, marshes and rivers. By day they hide low in the grass; after dark they climb vegetation and take flight, flashing to find a mate. Their light comes from a chemical reaction in the abdomen, and species-specific timing helps partners recognise the right signal. Their young live a quieter, hungrier life: larvae are carnivores that feed on soft-bodied prey such as snails and slugs. Adults may eat insects, nectar or pollen.

In Japan, watching them—hotaru-gari—has deep roots. Heian-era (794–1185) writers like Sei Shōnagon wrote about fireflies, and later literature and ukiyo-e art—a traditional form of woodblock prints and paintings—used their brief light to evoke impermanence.